Hepsin is a membrane serine protease expressed in several human tissues including the liver, kidney, prostate, and thyroid. The physiological function of hepsin remains unknown. In vitro studies have shown that hepsin activates blood clotting factors VII, XII, and IX, pro-urokinase (pro-uPA), and pro-hepatocyte growth factor (pro-HGF). Recently, hepsin has been identified as one of the most up-regulated genes in prostate cancer. The hepsin up-regulation appears to correlate with the disease progression. In a mouse model of prostate cancer, hepsin overexpression promotes cancer progression and metastasis. In culture, anti-hepsin antibodies inhibited the invasion of human prostate cancer cells. This review will outline the molecular biology and biochemistry of hepsin and highlight recent data of hepsin in prostate cancer.